The present invention relates to a heat-resistant lining for a gas turbine, consisting of a mat formed of ceramic fibers which is provided on its side facing the hot gas with a metal oxide-containing layer.
Gas turbines, as are provided in particular for use in the motor vehicle industry, must be operated for economy reasons at very high process temperatures that lie at about 1,300.degree. C. and higher. The supporting metallic parts of the turbine housing must be thermally insulated against these high temperatures.
A heat-resistant insulation is known from the DE-OS No. 26 30 247 which is formed of a ceramic fiber-mat on which a metallic oxide layer is applied on its side facing the hot gases. However, it is known from the DE-OS No. 34 46 649 that this lining does not possess a sufficient thermo-shock resistance so that during operation cracks occur in the lining which at the very high mechanical energy of the flowing gases very soon lead to damages of the lining.
The present invention is therefore concerned with the task to so further develop a heat-insulating lining for a gas turbine, consisting of a mat formed of ceramic fibers which is provided on its side facing the hot gas with a metallic oxide-containing layer, that no damages can occur any longer in the lining under alternate thermal stressing.
The underlying problems are solved according to the present invention in that the mat formed of ceramic layer has a gross density of 350 to 450 kg/m.sup.3 and the fibers in the mat which are in contact with the metal oxide-containing layer, are to be predominantly so oriented that the longitudinal axis of the fibers forms an angle of 30.degree. to 90.degree. with the metal-oxide-containing layer.
The danger of the occurrence of stress cracks and therewith of the destruction of the lining is eliminated in that, on the one hand, one utilizes a ceramic fiber mat which possesses a gross density of 350 to 450 kg/m.sup.3 and in that, on the other, the fibers in contact with the metal oxide-containing layer are predominantly so oriented in the mat that the longitudinal axis of the fibers forms an angle of 30.degree. to 90.degree. with the metal-oxide-containing layer. It is most favorable if the metal-oxide-containing layer is applied on the end faces of the fibers, i.e., if the longitudinal axis of the fibers forms about an angle of 90.degree. with the metallic oxide-containing layer.
The usual high temperature-resistant fibers, also with colloidal silicic acid as binder, can be used as fiber material. It generally involves fibers from the system Al.sub.2 O.sub.3 . SiO.sub.2 with very high Al.sub.2 O.sub.3 contents. They are made depending on the Al.sub.2 O.sub.3 -content either by air-blasting or jet treatment of silicate melts (aluminum silicate fibers) or are spun from solutions (mullite-, aluminum oxide fibers). Aluminum silicate fibers can possibly be stabilized with slight quantities of Cr.sub.2 O.sub.3. The gross density of the fiber mats formed from these fibers is to be between 350 and 450 kg/m.sup.3. If one drops below this density, then the strength is reduced as well as the coating ability of the fiber material, whereas if one exceeds the density of 450 kg/m.sup.3, then the heat conductivity of the fiber material increases and the temperature change resistance decreases. Preferred is a mat with a gross density of about 400 kg/m.sup.3. The thickness of the mat can be considerable depending on the technical exigencies and frequently amounts to 30 mm and more.
The mat is manufactured with such a fiber orientation that the metal-oxide-containing layer can extend essentially at an angle of about 90.degree. to the longitudinal axis of the fibers. Owing to this binding of the metal oxide-containing layer at the end faces of the fibers, a particularly good thermo-shock resistance is attained. However, it has also been found that a sufficient thermo-shock resistance is still attainable up to an angle of about 3.degree.. If the angle becomes more flat, then a sufficient endurance limit or strength of the lining is no longer attainable.
Coatings from the system Al.sub.2 O.sub.3 . SiO.sub.2 are usable in the first instance as material for the metal oxide-containing layer insofar as they possess the requisite temperature resistance. Still other oxidic components in different mixing ratios may be admixed to the main components Al.sub.2 O.sub.3 and possibly SiO.sub.2 as stabilizers or binders. As a result thereof, different properties of the coating, such as, for example, thermal expansion, or fusing temperature can be varied within a considerable band width and can be matched to those of the fiber material. It may be advantageous in some situations to draw the metal-oxide-containing layer additionally about the edges of the mat so that only the backside of the mat remains non-coated.
The metal oxide-containing layer can be applied on the fiber mat according to different methods. According to the plasma-jet-spraying process, particularly simple heat-proof or non-combustible layers can be made of mullite-, spinell-, and Al.sub.2 O.sub.3 powders. The powder particles are well fused in the plasma jet, are relatively highly fluid and can form a good interlocking with the fiber material upon the impingement on the fiber mat. The same is true for the flame spraying in which the same materials can be used.
In addition to these thermal spray methods, the metal oxide-containing layer may also be applied as glaze. Glazes can be built up of many oxides. They form during the baking a liquid phase which, after the solidification, adheres by wetting on the base material. By reason of the good chemical compatibility with the ceramic fiber mat, layers from the system Al.sub.2 O.sub.3 . SiO.sub.2 are preferred. Depending on the coating method, spraying or dipping, the glaze penetrates more or less deeply into the fiber mat. As the coatings are to be resistant at temperatures of up to 1,250.degree. C. and higher, the use of pure frit glazes is not suitable. More specifically, they contain, as a rule, larger quantities of B.sub.2 O.sub.3 which evaporates above 1,200.degree. C. Similarly, larger proportions in alkalies are to be avoided in the glaze because they contribute to a softening of the glaze already at low temperatures. By reason of the chemical compatibility admixtures of PbO and ZnO in the glaze are to be avoided because reactions are to be expected with the fibers in the contact area.
In addition to the classic glazes also so-called Engobes can be used. Engobes are clay glazes which still continue to be porous after the baking and exhibit hardly a glaze or shininess. They consist of a mixture of Kaolin chamotte and a silicate glaze as binding agent. A suspension of these two components is applied on the fiber mat as paste or slip and is baked on the mat after the evaporation of the suspension agent, generally water.
A further possibility to manufacture the metal oxide-containing layer which is to protect the fibers of the fiber mat against the erosion by the flowing gases, consists in coating the same with Al.sub.2 O.sub.3 by means of a clay-bond or alumina slip and to bake the same subsequently. An advantage of these clay-bond or alumina coatings consists in that the base body, namely the fiber mat and the cover layer have similar chemical and physical properties.
Phosphate-linked alumina slips set well already during the drying so that the coatings have sufficient gross breaking strength exist. The aluminum phosphate AlPO.sub.4 acts as so-callcd high temperature binder and imparts to alumina containing masses a high abrasion resistance. The clay-bond or alumina slip can be applied particularly favorably on the fiber bodies by brushing. In addition to the phosphate-linked alumina slips, also silicic acid-linked alumina slips can be utilized. An advantage with the use of colloidal silicic acids as binder resides in that the coating can be coordinated to the Al.sub.2 O.sub.3 . SiO.sub.2 system and the desired mullite formation at the contact places can be favored. In addition to Al.sub.2 O.sub.3 and SiO.sub.2, the slip may also contain MgO, as well as slight quantities of CaO and K.sub.2 O.
The manufacture of fiber mats suitable for the coating, in addition to making the same by pressing, can also be realized in that the suspending liquid is sucked off from a suspension of fibers. During the suction operation, the fibers then orient themselves perpendicularly to the suction direction. With very thick fiber mats to be remitted, it may be of advantage if one utilizes initially a fiber mat in which the fibers extend as customary parallel to the surface and in that one then applies on this mat a second layer of fibers, for example, of different thickness, possibly under interposition of a binding medium layer, in which the fibers form an angle of 30.degree. to 90.degree. with the metal oxide-containing layer to be applied. The thickness of the metal oxide-containing layer must be matched depending on thermal expansion and elasticity module to the base material in order to prevent stress super-elevations in the composite structure. In general, layer thicknesses of more than 200 .mu.m have proved sufficient. Preferred are layer thicknesses of 200 to 500 .mu.m because they possess a very good strength.